Spinal cord injury (SCI) is a devastating injury, which can be caused by motor vehicle accidents, violence, and non-traumatic causes. These injuries often leave patients with lifelong paralysis, as well as some amount of fecal and urinary incontinence. SCI disrupts the blood-brain/spinal barrier and causes a neuroinflammatory response at the lesion site. The influx of inflammatory cells results in demyelination and secondary destruction of axons that managed to survive the initial impact. Monocyte infiltration after SCI occurs in a biphasic pattern. Recent data suggests that circulating blood monocytes that infiltrate the spinal cord in the first phase after injury are key players in causing axonal breakage and death. To investigate the role of hematogenous monocytes in inflammatory damage after SCI, we are utilizing biodegradable polymer poly (lactic-co-glycolic acid) immune- modifying microparticles (PLGA-IMP). 500-nm microparticles of PLGA are intravenously infused into mice after SCI. By virtue of their negative surface charge and 500-nm size, the microparticles are selectively taken up by circulating monocytes. These IMP-bound monocytes are sequestered in the spleen and no longer traffic to sites of inflammation. In preliminary experiments, I have shown that acute IMP administration after SCI in mice reduces the influx of inflammatory cells into the injured cord and results in significantly improved long term functional recovery. This proposal will test the hypothesis that hematogenous monocytes are major contributors to inflammatory neuronal death after SCI because they preferentially differentiate to M1 classical macrophages and this damage can be prevented by IMP treatment. In the first specific aim we will investigate the polarization bias of the infiltrating monocyte population during the biphasic inflammatory response. We will then determine the effects of IMP treatment on inflammatory microenvironment through quantification of cytokine/chemokine expression, macrophage polarization, and axonal regeneration. In the weeks following injury, as the body attempts to close the blood-spinal barrier, astrocytes react to form a dense glial scar. This scar creates a physical and molecular barrier to regenerating axons that attempt to grow through the lesion in the later stages of recovery. While IMP treatment may initially reduce tissue loss, an ideal treatment for SCI would also facilitate the extension of regenerating axons in the later stages of recovery. Peptide amphiphiles (PAs) are small molecules that can be designed to self-assemble in vivo to form a synthetic extracellular matrix that displays bioactive peptide sequences at tunable density on the nanofiber surface. We have previously shown that intraspinal injection of self-assembling PAs after SCI decreases glial scarring and improves functional recovery by supporting the elongation of regenerating axons. The second aim of this proposal will be to combine IMP treatment with PA gel injection to investigate the possibility of synergy between these two complementary therapies. The proposed studies will help to define the inflammatory effects of circulating monocytes after CNS trauma, and may result in the development of an innovative therapy for SCI with potential for clinical translation.